Multi-stage flash evaporator with means to induce hydraulic jump



Aug. 20, 1968 D. CANE ET AL MULTI-STAGE FLASH EVAPORATOR WITH MEANS TOINDUCE HYDRAULIC JUMP Filed May 24, 1965 STEAM IN r4919 SU FLOWPERCRITICAL suscmncm. FLOW WITNESSES D INVEIiiTOQSe omemc on W979iThomas J- lvbos 8 Karl A. Ko'rzor United States Patent 3,398,059MULTI-STAGE FLASH EVAPORATOR WIT MEANS TO INDUCE HYDRAULIC JUMP DomenickCane, Springfield, Karl A. Katzor, Drexel Hill,

and Thomas J. Rabas, Havertown, Pa., assignors to Westinghouse ElectricCorporation, Pittsburgh, Pa.,a

corporation of Pennsylvania Filed May 24, 1965, Ser. No. 458,243 6Claims. (Cl. 202-173) ABSTRACT OF THE DISCLOSURE This invention relatesto evaporators, more particularly to multi-stage flash evaporators, andhas for an object to provide an improved arrangement permittingoperation of the flash evaporation chambers with optional minimal depthof the distilland, yet substantial decrease in the possibility of vaporblow-by between adjacent chambers. The above is attained by providingmeans including orifice means and upstanding means positioned, and soarranged and proportional with respect to the length of the chamber thatthe phenomenon known as hydraulic jump is induced in the distillandstream, which phenomenon is effective to raise the height of the flowingstream in that portion of the chamber adjacent the orifice from belowcritical depth to above critical depth. Critical depth as referred to inthe specification and claims is a specific hydraulic term defining adepth at which the specific energy of a flowing liquid is at a minimumvalue.

Multi-stage flash evaporators are usually provided with a plurality offlash chambers disposed in liquid communication with each other byinterconnecting orifices provided in the bottom portions of thepartitions dividing the chambers. The liquid to be evaporated(distilland) is heated under pressure and then progressively directedfrom chamber to chamber through the orifices for partial evaporation ineach chamber. To effect the staged evaporation, the chambers aremaintained at progressively lower pressures (considered in the directionof liquid flow) and the vapor generated therein is condensed andwithdrawn from the apparatus.

In flash evaporators of the above type, the orifices are preferably inthe form of horizontally elongated slots and so proportioned that thedistill-and flow path closely approximates horizontal open-channel flowunder a sluice gate. In the design of such evaporators, several factorsmust be considered, first limitation of the height of the apparatus toreasonable dimensions, and second attainment of good thermal equilibriumin the apparatus. Both of these factors lead directly to a flow path forthe distilland through the chambers that approaches the minimum depth.However, when minimum depth of flow is designed for the possibility ofvapor blow-by through the orifice from a higher pressure stage to itsadjacent lower pressure stage is increased, since the distilland flowlevel may fluctuate sufliciently to uncover a portion of the orifice. Aswell known, the vapor blow-by phenomenon leads to a degradation ofavailable energy in the distilland with a resulting reduction in thermalefliciency of operation.

A primary object of this invention is to provide a multi-stage flashevaporator having improved thermal efficiency and equilibriumcharacteristics yet more economical to manufacture.

A further object of this invention is to provide a multistage flashevaporator arranged to provide a distilland flow path through the flashchambers that is maintained at substantially optimal minimum depth withsubstantial decrease in the possibility of vapor blow-by betweenadjacent chambers.

3,398,059 Patented Aug. 20, 1968 A still more specific object of theinvention is to provide a flash evaporator of the above type in whichthe distilland flow stream in the evaporation chambers is initially at adepth less than critical depth and means is provided to induce asubsequent hydraulic jump in the flow stream.

Briefly, in accordance with the invention there is pro vided amulti-stage flash evaporator having a plurality of flash chambers formedby top and bottom wall structure and horizontally spaced upwardlyextending parti tions and outer wall structure. The partitions areprovided with horizontally elongated orifices or slots arranged totransfer the distilland from chamber to chamber at a level or depthbelow the critical depth. Critical depth, as well known in fluidmechanics, is the depth at which the specific energy of a flowing liquidis at the minimum value.

An upstanding member disposed on the bottom -wall structure andextending transversely of the direction of liquid flow is disposedintermediate the upstream and downstream orifices of the chambers. Thevertical extent of this member (which may be a plate) is that requiredto increase the depth of the liquid flow from below critical to abovecritical and is effective to induce the phenomenon known as hydraulicjump. During this transition, the liquid flow is converted from lowdepth and high velocity (rapid flow) to greater depth and lower velocity(tranquil flow). Hence, the downstream orifice is maintained in asubmerged condition and the possibility of vapor blow-by issubstantially eliminated or at least considerably reduced.

The above and the objects are effected by the invention as will beapparent from the following description and claims taken in connectionwith the accompanying drawings, forming a part of this application, inwhich:

FIGURE 1 is a diagrammatic vertical sectional view taken on line II ofFIG. 2 and illustrating a portion of a multi-stage flash evaporatorhaving the invention incorporated therein; and

FIG. 2 is a transverse sectional view taken on line IIII of FIG. 1.

Referring to the drawing in detail, FIG. 1 shows a portion of amulti-stage flash evaporator 10 having a plurality of flash evaporationchambers 12, 13 and 14 disposed in side-by-side relation with each otherand defined by wall structure including top and bottom walls 15, 16,side wall 17, internal partitions 18, 19 and 20, and front and rearwalls 21, 22 (see FIG. 2). A greater number of flash chambers may beemployed, if desired, as well known in the art.

In the upper portion of the wall structure, a plurality of vaporcondensing chambers 24, 25 and 26 are provided by horizontally extendingtray-like shelf structures 27, 28 and 29 terminating short of the frontwall 21 and jointly therewith forming flow passages, such as flowpassage 30, for directing vapor generated in the flash chambers upwardlyinto the associated condensing chambers.

Each of the condensing chambers 26, 25 and 24 is provided withrespective heat exchange tube structures 32, 33 and 34 seriallyconnected to each other and a suitable external heater 35, known as atop heater, is connected to the tube structure 34. The top heater, asillustrated,

comprises a shell structure 36 having a heating fluid inletposedadjacent the bottom wall 16 and providing serial fluid flowcommunication between the flash chambers 12, 13 and 1.4, respectively.

As thus far described, the structure is substantially conventional andoperates in the following manner. The distilland or pressurized incomingliquid to be evaporated, such as sea water for example, is directed, asindicated by the arrow 48, successively through the heat exchange tubestructures 32, 33 and 34 to the top heater 35, where it is heated to itsmaximum or top temperature and then introduced into the first flashchamber 12, through the inlet 42. The flash chamber 12 is maintained atan ambient pressure value P that is lower than that of the incoming seawater. Hence partial vaporization of the sea water occurs as it flowsthrough the chamber 12 and the vapors thus generated flow upwardly, asindicated by the arrows 49, into the condensing chamber 24, wherein, inthe resulting heat exchange with the relatively cool heat exchange tubestructure 34, the vapor is condensed and falls onto the shelf 27 and thesea water flowing through the tube structure 34 is heated.

The unvaporized sea water flows through the orifice 44 into the adjacentflash chamber 13, where it is exposed to an ambient pressure P that islower than the pressure P with resulting additional flash evaporation.In a similar manner, the remaining unvaporized sea water flows throughthe orifice 45 into the third and still lower pressure (P flash chamber14 where it undergoes additional partial evaporation and is thendirected through the orifice 46 for subsequent flash evaporation inadditional and still lower pressure flash chambers (not shown).

The vapors 49a and 49b generated in the chambers 13 and 14, in a mannersimilar to the vapors 49 in the first flash chamber 12, are condensedwith concomitant heating of the incoming sea water in the heat exchangetube structures 33 and 34 and the condensate is collected in the shelves28 and 29, respectively. For expediency, the condensate collected in theshelves 27, 28 and 29 is directed through suitable apertures 51, 52 and53 and withdrawn from the evaporator, as indicated by the arrow 54, aspure water for useful purposes.

In accordance with the invention the slotted orifices 44, 45 and 46 areso proportioned that the distilland flow stream is initially admittedinto the associated evaporation chambers 12, 13 and 14 at a depth lessthan the critical depth of the stream and at a greater than criticalvelocity. Also, since the properties of saturated steam and water aresuch that the pressure drop between the flash chambers is less than theprevailing chamber pressures P P and P the cross-sectional area of theorifices 44,

45 and 46 vary in size although they are all substantially the samelength and extend from the wall 21 to the wall 22. For example, therelation of the orifices is such that the orifice 44 has an area andhence a height smaller than that of the orifice 45 and the orifice 45has an area and hence a height that is smaller than that of the orifice46. Under such conditions, the flow stream immediately upstream of theassociated orifice, for example, the orifices 45 and 46, is barelysuflicient to cover the orifice, so that an unstable condition existswherein the orifices may or may not perform as sluice gates or submergedorifices. When they are not submerged, vapor from the upstream chamber,for example the chamber 13, is free to blow-by or leak past the orificeinto the immediately downstream chamber 14 because of the lower chamberpressure P; prevailing therein. This effect is highly undesirable, sincethe available energy in the distilland is degraded and the thermalefliciency of the evaporator is reduced. However, this effect is notsignificant in the first stage 12, since the distilland admitted.thereto by the spray inlet 42 is at the highest pressure level existingin the evaporator and is maintained at a safe level 56 adequate tomaintain the orifice 44 in properly submerged condition.

To overcome the above instability and thermal inefliciency duetoblow-by," the chambers 13 and 14 are provided with plate members 57and 58 extending upwardly from the bottom wall 16 and extending acrossthe entire width of the chambers, i.e. from the front wall 21 to therear wall 22,'The plates are vertically disposed in FIG. 1, however,they may be inclined in either upstream or downstream direction. Thevertical height of the plate members 57 and 58 is selected toinduce thephenomenon known as hydraulic jump, i.e. to convert the subcritical toabove critical flow, thereby to increase the depth .of flow downstreamof the plate members to a height above the critical depth and convertingthe high velocity subcritical flow to low velocity or tranquil flow.

Preferably, the height of the plate members is substantially equal tothe critical depth of the distilland stream and the plate members arepositionedupstream of their associated orifices a distance less than thelength of the so induced hydraulic jump, thereby insuring submergedoperation of the orifices and substantially eliminating the possibilityof blow-by of vapor.

The following specificexample employs figures and data attained with theinvention under typical operating conditions, considering chamber 13:

P =.96 p.s.i.a. at a saturation temperature T =l00.3 F. P =.70 p.s.i.a.at a saturation temperature T :90.0 F. L =15 ft. (length of the chamber13) w:3.25 ft. (width of the chamber 13) F=3.25 ft. (length of theorifices 44 and 45) By employing suitable known hydraulic equations, thefollowing data is obtained:

Critical flow depthY in the chamber 13:4.8 in.

The height H of the orifice 44:3.27 in.

The subcritical flow depth Y =l.97 in.

The theoretical length of the hydraulic jump=4 /e ft.

-Since the height H of the plate 57 is determined by 'Y and Y in thisexample, H is greater than Y and no greater than Y and may range fromabout 2 to 4.8 ins.

Also, since the length of the hydraulic jump is equal to 4 /6 ft. itwill now be seen that the plate member 57 may be placed a distance Lequal to but preferably less than 4 /6 ft. from the partition 19, butnever at a greater distance, in this example.

Data corresponding to the above may also be obtained for the flashevaporation chamber 14 and subsequent downstream chambers (not shown) toinsure the occurrence of the required hydraulic jump in these chambers.Although only one embodiment of the invention has been shown, it will beobvious to those skilled in the art that it is not so limited, but issusceptible of various other changes and modifications without departingfrom the spirit thereof.

We claim as our invention: 1. A multi-stage flash evaporator forevaporating a distilland flow stream therein having a bottom wallstructure and spaced upwardly extending first and second partitions atleast partly defining first, second and third flash evaporationchambers,

means defining a first horizontally elongated orifice adjacent thebottom of said first partition,

means defining a second horizontally elongated orifice adjacent thebottom of said second partition,

said first orifice being eflective to permit flow of unevaporated liquiddistilland from said first chamber to said second chamber for partialevaporation and to attain a depth less than hydraulic critical depth insaid second chamber,

said second orifice being effective to permit unevaporated liquid toflow from said second chamber to said third chamber for furtherevaporation, and

a member extending upwardly from the bottom Wall structure of saidsecond chamber and disposed intermediate said first and second orifices,

said member extending substantially transversely to said distilland flowand having a vertical height that is substantially equal to thehydraulic critical depth of the liquid flow in said second chamber andeffective to induce a hydraulic jump in said fiow that is of a heightgreater than said hydraulic critical depth, said first orifice being soproportioned that the distilland flow stream is initially admitted intothe second evaporator chamber at a depth not greater than the criticaldepth of the stream with a velocity not less than the critical velocity.

2. The structure recited in claim 1 wherein the member is a plateextending transversely to the direction of flow of the liquid andefiective to modify all of the flow,

the second orifice is of greater area than the first orifice,

and

the plate induced hydraulic jump attains a height greater than theheight of the orifice and is effective to prevent blow-by of vapor fromthe second chamber to the third chamber through the second orifice.

3. The structure recited in claim 1 wherein the second orifice is ofconsiderably less height than the depth of the hydraulic jump, and

the hydraulic jump inducing member is spaced up stream from the secondorifice a distance less than the length of the hydraulic jump, therebyinsuring that the second orifice is maintained in a submerged stateduring operation.

4. A multi-stage flash evaporator for evaporating a distilland flowstream therein having a bottom wall structure, front and rear walls andspaced upwardly extending first and second partitions eX- tending fromsaid front to said rear wall and at least partly defining first, secondand third evaporation chambers,

means defining a first orifice of substantially rectangular shapeextending the full width .of said first par tition,

means defining a second orifice of substantially rectangular shapeextending the full width of said second partition,

said first orifice being effective to permit flow of unevaporated liquiddistilland from said first chamber to said second chamber for partialevaporation at a hydraulic subcritical depth in said second chamber,

said second orifice being efiective to permit unevaporated liquid toflow from said second chamber to said third chamber for furtherevaporation at a depth less than hydraulic critical depth in said thirdchamber, and

a plate member extending upwardly from the bottom wall structureintermediate said first and second orifices and extending to said frontand rear walls,

said plate member having a vertical height that is at least greater thansaid hydraulic subcritical depth and at most equal to the hydrauliccritical depth of the liquid flow in said second chamber and eflectiveto induce a hydraulic jump in said hydraulic subcritical flow that is ofa height greater than said hydraulic critical depth, said first andsecond orifices being so proportioned that the distilland flow stream isinitially admitted into the associated evaporator chambers at a depthless than the critical depth of the stream with a velocity greater thanthe critical velocity.

5. The structure recited in claim 4 wherein the second orifice is ofless height than the hydraulic critical depth of the flow in the secondchamber, and

the plate induced hydraulic jump attains a height greater than theheight of the second orifice and is effective to prevent blow-by ofvapor from the second chamber to the third chamber through the secondorifice.

6. The structure recited in claim 4 wherein the plate induced hydraulicjump is of greater depth than the height of the second orifice, and

the plate is spaced upstream from the second orifice a distance lessthan the length of the hydraulic jump, thereby insuring that the secondorifice is maintained in a submerged state during operation.

References Cited UNITED STATES PATENTS 2,759,882 8/1956 Worthen et al202173 X 2,944,599 7/1960 Frankel l592 3,152,053 10/1964 Lynam 2021733,161,558 12/1964 Pavelic et al 202-173 X 3,172,824 3/1965 Mulford202-173 3,180,805 4/ 1965 Chirico 202-173 3,197,387 7/1965 Lawrance202-173 FOREIGN PATENTS 831,478 3/1960 Great Britain.

OTHER REFERENCES Fluid Mechanics (1937), Dodge and Thompson, 1stedition, McGraw-Hill, pages 242, 243, 248, 249, 250, and 251.

NORMAN YUDKOFF, Primary Examiner. F. W. DRUMMOND, Assistant Examiner.

